The present invention relates to optical imaging devices and imaging methods for microscopy. The invention can be applied in connection with the inspection of arbitrary surfaces or bodies.
In many technical areas, it is necessary, among others, to subject bodies and their surfaces to a precise optical inspection in order to be able, for example, to assess the quality of a production process and, where applicable, intervene correctively insofar as the inspection reveals that specified quality criteria are not fulfilled. Naturally, the same if not higher requirements must be imposed on the precision of the imaging device used for the inspection in comparison with the devices used for the production process of the body to be inspected.
In this context, the ability of the imaging device used for the inspection to process light of different wavelengths with minimum optical aberration is of particular importance in order to ensure a broad application field for the imaging device. In particular, in connection with the production methods which comprise an optical process, it is desirable or advantageous if the imaging device used can process the wavelength range also used during the optical process with minimum aberrations. This is, for example, the wavelength range of 193 nm (so called VUV range) to 436 nm (so called Hg g line).
A problem here is the chromatic aberrations i.e. the aberrations dependent on the wavelength of the light. If, for example, an imaging device with refractive optical elements (such as lenses or the like) is used for inspection, the aberrations of the imaging device are minimised at acceptable cost usually only for a comparatively narrow wavelength range. A so-called achromatization of such an imaging device comprising refractive optical elements, i.e. elimination of such chromatic aberrations, is scarcely possible with acceptable cost over a broadband wavelength range (such as that recited above).
Frequently, so-called catadioptric imaging devices are used which, apart from refractive optical elements, also comprise reflective optical elements. The disadvantages of refractive systems described above, however, also apply to such catadioptric systems as known for example from DE 10 2005 056 721 A1 (Epple et al.), U.S. Pat. No. 6,600,608 B1 (Shafer et al.), U.S. Pat. No. 6,639,734 B1 (Omura) and U.S. Pat. No. 5,031,976 (Shafer), the entire disclosure of which is hereby included herein by reference.
One possibility of largely avoiding the problems associated with chromatic aberrations is to use so-called catoptric systems in which exclusively reflective optical elements (such as mirrors or the like) are used for the imaging device. Examples of such catoptric systems are known from EP 0 267 766 A2 (Phillips), U.S. Pat. No. 4,863,253 (Shafer et al.) and US 2004/0114217 A1 (Mann et al.), the entire disclosure of which is hereby included herein by reference.
The problem with these known catoptric systems, however, is that for a desirably large magnification to be achieved with as few optical elements as possible, in particular, for the optical elements close to the object, comparatively large individual refractive powers are required. However, in view of the aberrations generated with such catoptric systems, this is disadvantageous so that, frequently, preference is given to the use of more than four mirrors, as it is known from US 2004/0114217 A1 (Mann et al.), or smaller magnifications or greater aberrations must be accepted.
If, in the system from US 2004/0114217 A1 (Mann et al.), only four mirrors are used, then a comparatively low numerical aperture of NA=0.7 can be achieved. Also, the mirror spatially closest to the object has a comparatively large diameter, which leads to a comparatively large thickness dimension and hence a greater working distance (between reflective surface and object surface).
A further problem in this context is the minimum obscuration at simple manufacture of the imaging device. In many systems the central passage apertures in the mirrors must therefore be designed conically in order to achieve minimum obscuration. Such conical passage apertures are however comparatively complex to produce, so that, as a result, the cost for the imaging device rises considerably.
In this context it is known from EP 0 267 766 A2 (Phillips) to use, instead of the conventional imaging devices with four mirrors each having a passage aperture, a system with two optical element groups, each of which comprises one convex mirror without passage aperture and one concave mirror with passage aperture. Here too, the problem is that, on the one hand, a very low numerical aperture (NA=0.3) can be achieved and that the mirror closest to the object again has a very large diameter.